<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=\, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    xxxxx
</body>
<script>
    var a = [1,2,3,[4,5,[5,6,7]]]
    var flatten = (arr)=> {
        return arr.flat(Infinity)

    }
    console.log(flatten(a))




















    // var a = [1,2,3,[4,5,6]]
    // var flatten = (arr) => {
    //     return arr.flat(Infinity)
    // };
    // console.log(flatten(a))
</script>
</html>